Debra Zimmerman, age 12, of Rochester, N.Y., for her question:

How can cactus plants grow in a dry desert?

Plants take liquid nourishment and their chemical foods must be dissolved in water. Water seeps in through the roots, circulates through the cells and evaporates out through the leaves. Most plants need a constant supply of rain, but a few have ways to conserve and store water. They can live in the desert where showers are few and far between.

Most cactus plants have shallow roots that spread over a wide area. When a rare shower falls, they can soak up the water before it runs away. They have swollen stems with pores to store water until the next shower arrives. Water cannot Evaporate through their surfaces, for they have thick skins. Such specialized plants can live and thrive through long periods of  drought.